Numerous nondestructive testing techniques are available for detecting flaws or discontinuities in the surface of a material. Many of these testing techniques are based on the principle of applying a liquid penetrant (e.g. a dye) over the surface of the material and allowing the dye to penetrate into flaws and discontinuities in the surface of the material by capillary action. These techniques typically require the use of a development agent which is applied over the dye and acts as a blotter to assist the natural seepage of the penetrant out of the flaws or discontinuities within the material. After the surface has been sufficiently "developed", the surface is visually examined for indications of penetrant "bleedback" from surface flaws or discontinuities.
A typical prior art flaw detection technique consists of five basic steps:
(1) surface preparation; PA1 (2) penetration; PA1 (3) removal of excess penetrant; PA1 (4) development; and PA1 (5) inspection.
In the first step, the surface of the material to be inspected is thoroughly cleaned to remove any oil, water or other contaminants from the surface of the workpiece. The resulting cleaned surface is then thoroughly dried. In step 2, the liquid penetrant is applied over the surface of the workpiece and is allowed to remain on the surface for a period of time long enough to allow the penetrant to penetrate into any flaws or discontinuities that are present in the surface. In step 3, any excess penetrant is removed from the surface of the workpiece. The method used to remove the excess penetrant is determined by the type of penetrant used. Some penetrants can be simply wiped off or washed away with water, whereas other penetrants require the use of solvents. In step 4, a thin layer of a powder-like substance is applied to the surface of the material. The powder-like substance acts as a blotter to assist the natural seepage of the penetrant remaining in the surface flaws or discontinuities resulting in a visual indication of the flaw or discontinuity. The final step in this process involves a visual inspection of the surface to detect the flaws or discontinuities made visible by the penetrant and the blotting action of the powder-like substance. The foregoing procedure is very time consuming and relies on the ability of the inspector to detect the surface flaws and discontinuities made visible by the penetrant and the powder-like substance.
Lorenzi (U.S. Pat. No. 3,786,346) discloses a method for detecting defects in a magnetizable test piece using magnetic particles in a viscous fluid. As such, the method disclosed in this reference comprises the steps of spreading a relatively viscous slurry of ferromaganetic flakes over the surface of the test piece, applying a magnetic field to the test piece to orient the ferromagnetic flakes, and inspecting the test piece to determine the existence of flaws within the piece. The application of the magnetic field causes the ferromagnetic flakes adjacent to a flaw to rotate so that their edges are directed toward the viewer causing the flaw to show up as a dark line against a gray background.
Mlot-Fijalkowski, et al (U.S. Pat. No. 4,433,289) discloses a method for testing magnetizable workpiece which comprises the steps of applying a dispersion of ferromagnetic particles in combination with a fluorescent pigment and water soluble carrier on the workpiece to be tested, applying a magnetic field to the workpiece, applying an aqueous spray to the workpiece, drying the workpiece, and examining the workpiece under ultraviolet light.
The above flaw detection techniques have numerous inherent disadvantages in that performance of all the required steps is very time consuming and relies heavily on the visual ability of the inspector to properly examine the material for surface flaws of discontinuities. Furthermore, the techniques of Lorenzi and Mlot-Fijalkowski require magnetizable workpieces.
Additionally, it is known to utilize a ferrofluid (a stable colloidal suspension of sub-domain magnetic particles in a liquid carrier) in determining wall thickness and hole (cooling channel) size in turbine blades in conjunction with an eddy current inspection instrument. It must be recognized, however, that such a technique is drawn to the measurement of relatively large internal features that the user knows are present, in contrast to an inspection of the surface of a workpiece to determine the presence of potentially very "tight" flaws or cracks that may or may not be present.
Because of the foregoing, it has become desirable to develop a flaw detection system which minimizes the amount of time required for the testing procedure and which does not depend upon visual inspection for the detection of surface flaws or discontinuities in the surface of the material.